12v 250mA limited supply, with output overvoltage protection

[oddbudman]

psu 12vdc 250ma

Afternoon all!

I've been stumped on this one for a while now, perhaps someone on this forum can help me out.

I'm trying to design a special bus powersupply that puts out 12v DC with a current limit of 250mA. The tricky thing is its output needs fixed overvoltage protection.

ie if some bozo comes along and connects 240v AC to the supplys outputs the powersupply can't fail-even under prolonged connection.

I will reiterate the fact that i need to protection on the output of the supply - not the input.

Anyway, if anyone out there can offer any advice on how to skin this cat it would be greatly appreciated.

Thanks in advance,
oddbudman

 

[flatulent]

dali 250ma

One trick is to have the current limiting be of the "fold back" type which turns the output down to 0 V when the current limit is reached. Then you put a zener type circuit on the output which trips at 13 V in your case. If the mains connection is wrong the output tries to rise above 13 V and the over current circuit drops it to 0 V.

Another approach is to put MOV protectors on the input past the fuse. If the higher mains is applied, the MOV will conduct and blow the fuse.

 

[oddbudman]

output 12v 250ma

Wow, thanks for the fast response.

Can you explain your first approach a little more please?

As it stands my PSU uses a bias resistor that, if too much current flows though it, will turn on a transistor, that turns off the main transistor. I kinda think this could be what your elluding too, I'm just a little confused where the zener comes into play.

anyway, here is where I'm kinda at, any suggestions / additions / etc would be great.

thanks,
oddbudman

 

[IanP]

Re: 12v 250mA limited supply, with output overvoltage protec

First of all I think you do not need "current foldback" circuit, just current limited to 250mA.

The other thing is how to protect your output from 240Vac...
This is really tricki question because you have to fast cut off your output for as long as the dengerous voltage is present and then again fast re-conect it when the voltage is gone.
MOV will explode.. or blow a fuse (if there is one somevhere)..
Let us think of something, as even using fast relays (solid state in not good for DC) as a cut-off device you still will expose some components in your 12V power supply to 240Vac and they will be dead and there has to be detection circuit which always has some dalay.

 

[resgrt]

Re: 12v 250mA limited supply, with output overvoltage protec

can u explain the purpose of power supply may be an other better than..

 

[oddbudman]

Re: 12v 250mA limited supply, with output overvoltage protec

It's a DALI bus PSU.

Dali stands for Digitally Addressable Lighting Interface.

http://www.dali-ag.org/

Since DALI runs in parallel with all other mains wiring the protection is considered necessary - especially when you look at the likelihood of some bozo wacking a DALI PSU onto a bus that is running switchdim (switchdim uses ACTIVE/NEUTRAL signalling on the DALI terminals to fade up and down lighting control gear).

I've read many an application note on DALI and none I've found offer anything beyond transient voltage protection for the bus PSU.

 

[IanP]

Re: 12v 250mA limited supply, with output overvoltage protec

This is what I wanted to suggest to you: transient suppressor (bi-directional and 2 resettable fuses (eg MFR series) for both +V and 0 lines.
These fuses are very, very fast and will re-set once it hes riched its 'trip' temperature and can take a greater power pulse than ceramic PTC resistors.

 

[tjalps]

Re: 12v 250mA limited supply, with output overvoltage protec

Hello!

Does anyone have any good solution for DALI power supply?

Regards!